Hello, first I must say, what a great programme Space Engine is. It is one of the best programmes I ever seen, due to its ability to show the dimensions of space. But one little thing surprised me: In Space Engine M 42 has a distance of 110 pc to sol and a diameter of 2.2 pc. In reality it has a distance of ~ 414 pc and a diameter of ~ 9.1 pc. Also the ring nebula (M57) has a distance of 216 pc (700 pc in reality). More nebula have the same problem (Eagle nebula 658 pc : 2100 pc).

Is this a bug or a data base failure?

I also note, that in SE some gas giants have the same number of moons like a rocky planet with a much lower mass. I think, this is unrealistic, because often rocky planets have one moon or often no moon, in rare cases two. And gas giants have often more than 20 moons, because they have catched many asteroids with their gravity.

The number of moons should be linked with the mass of the planet and planets with high masses (0.5 Jupiter-masses) should have higher chances for asteroids as moons. This should take more realism into the solar systems in SE.

Except of these two points, Space Engine is a realistic and brilliant programme.

Maybe I'm using obsolete or incorrect data for nebulae. By the way, I need a good catalogue for known diffuse nebulas to add them into SE, so I can work with it later.

Gas giant planets must have a moon system with total mass of ~0.0001 of the planet's mass. This is empirical and theoretical value (the moons around gas giants are formed exactly like planets around sun, by accretion in gas-dust disk, surrounding a planet). In 0.94 there are some bugs in generation algorithms that lead to generation of too low in mass satellites. I've already fixed it.

Terrestrial planets can have moons formed by such mechanism, same applies with gas giants. In this case a planet's moons will be very small, assuming a 0.0001M mass value. But with a certain probability; a proto-planet will collide with another proto-planet that will lead to formation of giant moon, as our Earth-Moon system does (see giant impact theory). But in the 0.94 verson these scenarios are not implemented, I just assume maximum moon mass as 0.1 of a planet's mass, and generate several moons like it is for gas giant.